@ Philipp Kewisch: thanks for your quick response.I am using TB 15.0.1 and Lightning 1.7Updated 1 week ago.Lightning was working well until today.Yes, i had an error message today by a pop-up window. Something about Lightning, but unfortunately I clicked it away...

edit: problem solved!In the past I had linked Lightning with reminderfox, which I use in FF.In TB/Lightning I use the reminderfox calendar instead of private calendar of Lightning, and the setting of the reminderfox calendar was "write-protected". I really don´t know if the setting also was "write-protected" before today (I did not change it consciously), but after deleting "write-protection" everything is working well as usual now.Now then I gladly change my rating from 1 to 5 stars.